Member - 'Lucy' replied:

Greenant

I've never experienced a diesel do this but have a number of times with domestic, petrol engines with alloy heads.

Invariably it turns out to be a crack in the head through to an oil gallery. (sometimes reparable)

A gasket usually shows as a pressurised water jacket (bubbles etc) or water in the oil.

Rigor replied:

Don't overlook the heat exchanger oil/water as they are known to corrode through at times . No pressure bubbles or expanded hoses just water in the oil and oil in the water !

Dave L